Acrylic copolymers, a kind of polymers formed from ethylenically-unsaturated monomers (which may hereinafter be called simply “monomers”), are used in a variety of fields such as high-molecular dispersants, paints, binders, and self-adhesives. Many of these acrylic copolymers are produced by general radical polymerization. According to such general radical polymerization, however, a termination reaction such as the recombination of propagating radicals themselves or a disproportionation reaction takes place, thereby making it difficult to control the molecular weight distribution of the resulting polymer. Moreover, end radicals of the polymer are deactivated during the polymerization so that, even when another monomer is added and polymerized after the polymerization is conducted, the polymer cannot be formed into a block polymer.
Upon producing a pigment dispersion of high pigment concentration, on the other hand, it is difficult to stably disperse a pigment. During or after the production of a dispersion, diverse problems hence arise such as the aggregation or settling of pigment particles in the dispersion after its production. In general, a high-molecular dispersant is formed of a pigment adsorbing segment and a solvent compatible segment. The pigment adsorbing segment adsorbs on pigment surfaces, while the solvent compatible segment is soluble in or compatible with a dispersion medium. For the uniform dispersion and stabilization of a pigment in a liquid medium, it is necessary to use, as a high-molecular dispersant, a polymer having such segments arranged with a good balance relative to the pigment and liquid medium or a block polymer or graft polymer having the respective pigments strictly controlled relative to the pigment and liquid medium.
Especially in a block polymer having a strictly controlled structure, functional sites of a pigment adsorbing segment and solvent compatible segment are clearly separated from each other so that molecules of the block polymer adsorb on each pigment particle at multiple points to achieve stronger attachment. Further, solvent compatible segments extend in a solvent, and therefore, can stably disperse pigment particles owing to a steric effect. Generally, a polymer of a low-polarity alkyl(meth)acrylate is often used as the solvent compatible segment of the high-molecular dispersant, and as its pigment adsorbing section, a polymer of a (meth)acrylate having a polar group such as an acid group, amino group or amido group is often used.
In the conventional radical polymerization of an acrylic monomer, however, only a random copolymer is available. As a termination reaction takes place in the course of such polymerization as described above, no block polymer can be obtained, and therefore, it is difficult to form the above-described respective functional sites separately.
Processes, which make use of living radical polymerization, have thus been developed for the production of block polymers. Developed as specific examples include the nitroxide mediated polymerization (NMP) process that makes use of dissociation and bonding of amine oxide radicals (Non-patent Document 1), the atom transfer radical polymerization (ATRP) process that conducts polymerization in the presence of a halogen compound as an initiating compound by using a heavy metal such as copper, ruthenium, nickel or iron and a ligand capable of forming a complex with the heavy metal (Patent Document 1, Patent Document 2, and Non-patent Document 2), the reversible addition-fragmentation chain transfer (RAFT) process that conducts polymerization by using an addition-polymerizable monomer and a radical polymerization initiator in the presence of a dithiocarboxylate ester, a xanthate compound or the like as an initiating compound (Patent Document 3), the macromolecular design via interchange of xanthate (MADIX) process (Patent Document 4), the degenerative transfer (DT) process that makes use of a heavy metal such as an organotellurium compound, organobismuth compound, organoantimony compound, antimony halide, organogermanium compound or germanium halide (Patent Document 5 and Non-patent Document 3), and so on. Extensive research and development work is underway on the living radical polymerization process.
For example, a high-molecular dispersant has been produced by the NMP process (Patent Document 6). This high-molecular dispersant uses, as a polymer adsorbing segment, a polymer block of an amino-containing monomer such as N,N-dimethylaminoethyl acrylate, and as a solvent compatible segment, a polymer block of a hydrophobic monomer such as n-butyl acrylate.